I have tried all the usual security measures: Switch off firewalls, reboots, cache clearing, allowing installation from Developers and Apple in Prefs>Security, etc. Is there anything else that I may be overlooking?
Two things you did not specifically mention:
1./ Download the version you are trying to install direct from eMC
2./ Run the upgrade with eMC closed (exit eMC before running upgrade)
This often helps with Windows, I assume the same will hold good with MAC